Pinch-type valves with flexible tubular spool-sleeves are commonly used in the chemical process industry, food industry, mining, metallurgy, plating and indeed in any industry where liquids containing solids, such as slurries, and other hard-to-handle fluids and even solids are made to flow. Pinch valve devices that include spool-sleeves are used for both on/off and flow control situations. The general distinctions and advantages of pinch valves over other types of valves are well known and recognized in the art.
The basic construction of a pinch valve consists of a flexible resilient flanged spool-sleeve installed in a fluid process line together with means to constrict the tube so as to regulate, throttle or terminate the flow of the fluid through the line. Some pinch valves employ a mechanical pinch mechanism to constrict the spool-sleeve. One type of pinch valve manufactured today consists of a flanged spool-sleeve in which the spool-sleeve is fitted either into a one-piece housing (by distorting the flange and pushing the spool-sleeve into the housing) or more commonly, in a split housing with its associated sealing gaskets and bolts. The spool-sleeve flange is outside the housing and is sandwiched between flanges on the housing and the line piping and serves to seal the valve against leakage of process fluids. Where adjustment of the valve is by a motive fluid, for example air or hydraulic fluid, which squeezes the intermediate portion of the sleeve, the flange also serves to seal the motive fluid.
U.S. Pat. No. 4,682,755 illustrates a pinch valve that includes a housing with a quarter-turn closure mechanism disposed within the housing for adjustment of a spool-sleeve. The mechanism consists of two cam members rotatably supported in the housing on the opposite sides of the sleeve and connected to a hub via drive links. The hub is disposed proximate an opening in the housing and is rotated manually by a handle, or automatically by an actuator. Each cam member includes a first portion that is rotatable across the sleeve and a shank portion that is eccentrically disposed on the first portion and traverses the spool-sleeve profile. As the cam members are rotated the shank portions move transversely of the sleeve longitudinal axis, toward and away from one another for pinching the spool-sleeve and thereby selectively throttling or stopping the flow through the system. When rotated to a fully retracted or release position, the shank portions are completely withdrawn from the outside profile projection of the spool-sleeve, whereby the spool-sleeve can be removed axially from the housing. The pinch valve also may have motive fluid means for pinching the spool-sleeve. The device comprises a one-piece body design incorporating two closure cams operating in an arcuate path to close the sleeve, and a relatively complex drive linkage. Component and assembly costs made manufacture of this design uneconomical. To simplify the design, the drive linkage was replaced with a slotted cam plate assembly, in which the top of each cam was fitted with a pin and bearing that rode in a cam plate slot. Although more economical, this design yielded high torque values and mechanical problems associated with bearing point loading.
A major improvement then followed, wherein a vertically split body with gears affixed to the top of the cams, are driven by a center gear and stem assembly for rotating the cams, (i.e., a three-gear drive system). In addition, the cams were turned (or advanced) so as to decrease torque by decreasing cam angular travel. This system was suitable more for flow control applications, rather than for on-off situations. Since a majority of applications for this valve were found to be for on-off applications, the design was again modified to the present configuration, in which the cams are at a full open position, with the resultant increase in torque value. When viewing the valve waterway, a full circle is seen. This configuration is deemed best for liquid-solid particle slurries in on-off applications, although the valve is still able to be a reasonable flow control valve.
However, there exists a major problem with pinch-type of valves of the type described above. When the valves are closed for an extended time period, the sleeve will tend to set in the closed position. The sleeve can stick in the closed position, for example if the process material holds it together, or it may open slowly or only partially. In many situations, process pressure is high enough to overcome this set condition, at least in part, so as to sufficiently open the sleeve. However, when process pressure is low, as in gravity discharge from a tank, the sleeve may remain in a partially closed position. This situation is even more evident when the sleeve is lined with a plastic such as PTFE. The increased rigidity of the plastic-lined sleeve causes very slow recovery to the open position, even when this type of sleeve remains closed for only a short time.